I have a 1973 340 plymouth duster with a 727 trans. The 8 1/4- 3.21 rear end blew up so now I have installed an 8 3/4-3.91 with a 489 case and now it vibrates at 60 miles per hour. Does anyone know if the driveshaft should be replaced when you switch to a 8 3/4 differential? Is it a different length Any help would be appreciated. Jon  

340,did ya check yoke splines and count and also check trans splines? if it seem to fit and theres around 3/4 left before bottoming out it should be good to go. maybe the driveshaft needs a balance job, any noise? maybe your axle bearings are old ,maybe ya got a loose ujoint,or wron ujoints.maybe the gears are old and way out of tolerances, normally you would see a pinion seal leak first,maybe the carrier bearings seen better days,maybe you didnt set pinion angle when you switched rears,maybe bent driveshaft.put it on a post hoist and have someone sit in it while your running at the speed the vibrations coming from and watching the driveshaft closely,or its coming from another source, thats the easiest way, yes a lot of these maybe dont apply but any are possible  

Mopar, I am upgrading to yukon axles with the green bearings installed just waiting for some backing plates to upgrade from 10 x 1.75 to 10 x 2.50 drums so I am hoping this may help. I have the driveshaft in and it looks like about 1 1/4 to 1 1/2 of space so there may be the trouble. I have now found out that the axle centerline to the yoke centerline is greater for the 8 3/4. They say it is 12.35" for the 8 3/4 and 11.69" for the 8 1/4 for a difference of 0.66". Could the driveshaft being a little short be the cause of the vibration? Any advice on getting a driveshaft? The other thing you mentioned way the ring and pinion may be worn out.The gears did have some wear so this also could be the problem although they were installed by someone who knew what he was doing. Thanks for all the great advice. Jon  

the lift is the best way to determine where the vibration is coming from. heck yes your driveshafts too short,1 1/4 to 1 1/2 too short? thats trouble.3/4 inch max before bottoming out in case.gears will show signs of wear ,look at the pattern of wear and where the ring and pinion is engaging depth wise.its normal.on the driveshaft just get a B body driveshaft with the smaller u joint I assume your using and have it cut down and rebalanced ,remember to take the measurement allowing the driveshaft 3/4 before bottoming out in the case.take care  

Thanks for the great info. I sould disconnect my driveshaft to see how much space I have until it bottoms out into the trans yoke. Any other info would be appriciated.thanks,. Jon.  

your welcome, and thanks for being so kind...  

yes, check your pinion angle  

I totally agree this is a pinion angle issue. Very easy to check with a $5 angle guage. Just set the guage on the bottom of the driveshaft and note the angle. Then put the guage on the pinion yoke and note the angle. The pinion should be at a 5-10 degree downward angle in relation to the drive shaft for a leaf spring- 7 degree's is the preferred angle. My car was at about zero and a 6 degree shim made a world of difference. Before I did this I could hardly make a pass down the strip because of such a bad vibration. I have some 4 degree if you need them.  

make sure you check pinion angle with car on the ground. You can't check it with the rear axle hanging.  

Im glad you got it fixed dark.6 down. 5 to 7 down is the number. use an adjustable snubber and set the clearance of the rubber mount about 1/2 lower before it hits the floorpan on an automatic, set it just so it touches the floor, no ya cant set the angle in the air heh,did you have to change the rear shock shackle plates and use longer ubolts from the 8 1/4 and the 8 3/4? take care, were gettin there.I think you can figure out where the yoke is sitting in relation to the trans case and leaving 3/4 space between them, hint, just mark the yoke where it goes into the rear seal then take driveshaft off and check the difference by sliding the driveshaft til the yoke hits and remark,then check the difference. take care, glad its not shaking anymore.ps make sure your using at least 25 inch long shocks fully extended .  

using the 3/4 number and setting the pinion snuber is critical as when you accellerate your rear center will rise.you could leave 1/4 inch spacing on the automatic trans pinion snubber using the 3/4 number just to play it safe..take care  

post 562 should read ,set it just so it touches the floor on a 4 speed car, sorry  

forget the post number heh, set the 4 speed pinion as stated above at 2.43 pm lol. just woke up. lol  

Thanks for the great advice. They say you only have to change the u bolts when you are going from a 7 1/4 to an 8 3/4. Hey darktone at what speed were you getting a vibration? Jon  

I was getting a vibration about 75mph that would stay with the car until I slowed way down. I have talked with others though with pinion angle problems that has the vibration come in earlier.  Just talked to a guy at a car show with a 73 satellite that had a vibration start at about sixty and was pretty bad. I bent down and looked at the pinion and could tell right off it was about zero degrees- then he told me the problem started when he swapped the 8 1/4 for a 9 1/4. Do you have the soft ride rubber insulators?